Contact lenses in wide use today fall into two categories. First, there are the hard or rigid corneal type lenses that are formed from materials prepared by the polymerization of acrylic esters, such as polymethylmethacrylate (PMMA). Secondly, there are the gel, hydrogel or soft type of lenses made by polymerizing such monomers as 2-hydroxyethyl methacrylate (HEMA).
Solutions that wet the lenses before insertion in the eye are required for both the hard and soft types of contact lenses. After the contact lenses are inserted in the eye, ophthalmic solutions for rewetting, lubricating, and/or enhancing wearer comfort are sometimes applied to the eye by means of an eye dropper.
Wetting or rewetting solutions usually contain a wetting agent in combination with a germicide or preservative, a viscosity builder, and salts that adjust the tonicity of the solutions to make them compatible with the osmolality of tear fluids. Such a wetting solution, for example, is disclosed in U.S. Pat. No. 4,323,467.
The hard acrylic type of contact lenses are highly durable and, since they do not absorb appreciable amounts of water, the selection of a suitable disinfecting agent or other chemical agent for lens care is relatively non-critical. Unlike hard lenses, however, the soft type of contact lenses have a significant tendency to bind and concentrate the antimicrobial agents found in lens-care solutions. This tendency can be exacerbated by the inherent binding action of any protein deposits on the lenses. Soft lenses are more susceptible to protein deposits and, hence, more susceptible to the binding of various antimicrobial agents.
Thus, in spite of low toxicity levels, not all disinfectants are compatible for use with all types of contact lenses. Many disinfecting and preservative solutions for hard lenses contain benzalkonium chloride. Although it is an effective antibacterial agent, it is incompatible and should not be used with soft contact lenses, because it is taken up by the lens. Severe toxicity may occur due to sustained release of benzalkonium chloride from the lens. Thimerosal, or sodium ethylmercurithiosalicylate, was introduced as an alternative to benzalkonium chloride, but is also incompatible with soft lenses.
Antibacterial agents must be selected that do not build up on lens surfaces and do not become concentrated to potentially harmful levels, such that corneal inflammation or other eye irritation results. Previous efforts to alleviate the problem of binding and concentrating of disinfectants and preservatives onto contact lens surfaces and to reduce the potential for eye tissue irritation have not been totally satisfactory.
Sorbic acid is less irritating than mercurials and has experienced some popularity as a preservative. However, it reportedly may increase age-associated yellowing of some lenses, particularly those containing methacrylic acid or the like.
Various biguanides have been found to exhibit good compatibility with soft contact lenses and have been used as a germicide in some contact-lens solutions. U.S. Pat. No. 4,758,595 discloses the use of biguanides in a multi-purpose contact-lens cleaning solution that does not include any polymeric demulcents. U.S. Pat. No. 4,786,436 discloses a wetting solution which discloses the use of a biguanide or water soluble salt thereof, in combination with collagen and other demulcents such as hydroxylethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxylpropylcellulose and the like. However, whether due to instability or incompatibility of the biguanide in general eye care formulations, or for some other problem or deficiency, biguanides have not been commercially used in eye-drop solutions. Some past studies have suggested that the biguanides known as PAPB or PHMB (polyhexamethylene biguanide or its salt) exhibited poor stability or loss of preservative efficacy in small volume containers. Originally, it was thought that this problem may have been due to an undesirable interaction of the biguanide with the plasticizer present in the LDPE (low density polyethylene) material used to make squeeze bottles that typically hold eye-drop solutions.
It is also known that biguanides may undesirably react with certain polymers in solution. For example, Kennedy, J. F. et al., "The Assay of Acidic Polysaccharides in Solution with Poly(Hexamethylenebiguanidinium choloride)," Carbohydrate Research, 156 (1986) at 79-85 discloses that the biguanide known as PHMB complexes with and even precipitates carboxylated and sulphated polysaccharides.
A variety of polymers are known as demulcents in the field of ophthalmic products, including carboxymethyl cellulose sodium, hydroxyethyl cellulose and other cellulose derivatives, dextran, gelatin, and polyols such as glycerin, polyethylene glycol, polysorbate, propylene glycol, polyvinyl alcohol, and povidone. U.S. Pat. No. 4,120,949 discloses the use of polyvinylpryrrolidone, or povidone, as a demulcent. Rankin, in U.S. Pat. No. 3,920, 810, discloses the use of polyvinylpyrrolidone (PVP) as a demulcent and lubricant in solutions used to treat dry eye. PVP is known to act as a demulcent lubricant by means of a combination of adhesive and lubricating properties that aid in the spreading of its viscous solution. PVP, however, is also known to be incompatible with a wide variety of inorganic salts and other chemicals. See, for example, Wade, A. et al, ed., Handbook of Pharmaceutical Excipients, 2d Ed. (American Pharmaceutical Association, Washington 1994) at p. 396. The efficacy of some preservatives, e.g. thimerosal, may be adversely affected by the formation of complexes with PVP.
It would be desirable to provide an eye-drop solution for wetting and lubricating lenses that is formulated using a biguanide preservative, without any loss of efficacy, thereby allowing the replacement of sorbic acid or benzalkonium chloride with a biguanide which is inherently less irritating to the eye. Furthermore, it would be desirable to provide an eye-drop solution that can be safely and efficaciously used for both lubricating contact-lens in the eye and, irrespective of whether contact lenses are being worn, for treating dry eye or for soothing eye irritation.